City’s starting XI included three changes from Monday’s 1-1 draw with Blackburn Rovers, with Viktor Johansson, Mitchell Clark and Darnell Johnson coming into the home line-up.
It was the Seagulls who threatened first on Filbert Way as an injection of pace from Aaron Connelly down the right wing allowed him to find James Tilley in the penalty area.
While his shot was on target, aiming for the bottom corner, Sweden Under-21s goalkeeper Johansson did brilliantly to get his leg to it and was able to keep the ball out on three minutes.
With 18 minutes gone, George Cox galloped down the left channel and located Aaron Connelly in the box who, after holding off Filip Benković, slid it into Peter Gwargis.
Although it appeared destined to ripple the back of Leicester's net, the Australian’s low effort was saved – and held – by the authoritative arms of Johansson.
At the other end, Callum Wright and George Thomas both had shots blocked for City’s youngsters, but it was the visitors who were creating the most likely chances in front of goal.
That trend continued just before the half-hour mark when Cox once again made ground on the flank and crossed for Connelly, but the PL2 Player of the Year couldn’t apply a finish.
The Seagulls got the goal their early showing deserved before the break when Connolly applied a brilliant curling finish to a quick break, putting City behind for the first time this year.
Ali Reghba, meanwhile, who scored on Monday, was forced to withdraw soon after, with former Oud-Heverlee Leuven striker George Hirst making his maiden outing for Leicester.
The Englishman was unlucky not to level the arrears before the interval, too, when his flicked header – on a smart George Thomas cross – clipped over Jason Steel’s crossbar.
City had Sam Hughes among their ranks in the second period, replacing Croatia international Filip Benković, but the home side were soon two goals down.
On 50 minutes, Seagulls midfielder Jordan Davies capitalised on a loose ball and slid it in for Peter Gwargis to apply the finish beyond Johansson in the Foxes net.
As the Foxes went in search of a goal which would have provided them with hope of a comeback, they were awarded a free-kick from around 25 yards out with 18 minutes to go.
It was Hirst who stepped up to take it and Steel had to scramble across his line to get a hand on the low drive, which was flying into the bottom corner, if not for a late intervention.
Substitute Callum Wright, on the other hand, was also unfortunate when he latched onto Clark's cross, but the former Blackburn Rovers man could only find the side netting.
It got worse for City, though, with two minutes to go as Bojan Radulovic Samoukovic received the ball from Ryan Longman to make it 3-0 late on.
Major moment…
Conceding the second goal, so soon after both sides had emerged from the tunnel for the second half, ultimately gave City too tall a mountain to climb in the latter stages.
The Foxes will be disappointed by the nature of Peter Gwargis’ effort, too, but it put breathing room between them and Brighton, who saw out the match with composure on Filbert Way.
Who impressed?
Speaking on LCFC TV, former Foxes manager Micky Adams said: "I think that, since he got on the pitch, he’s had about three efforts on goal and hit the target twice. I think there’s more to come from him. My Man of the Match from a Leicester City perspective is George Hirst."
Where does it leave us?
Leicester's youngsters drop to ninth in Premier League 2 Division 1 following Friday's round of fixtures, with two points from three games.
Coming up…
The Foxes travel to New Bucks Head, the home of AFC Telford, to tackle Wolverhampton Wanderers in Premier League 2 Division 1 next Friday evening (7pm BST kick-off).
As usual, the clash will be available live and for free on LCFC TV with commentary from Nick Walsh and former Leicester City and Republic of Ireland defender Sean St Ledger.
The details…
City: Johansson; Clark, Benković (Hughes 45’), Johnson, Ughelumba; G. Thomas (c), Dewsbury-Hall, Leshabela (Shade 62’), Wright; Muskwe, Reghba (Hirst 41’)
Subs not used: Davies, L. Thomas